The present invention relates to methods of forming an insulating film and methods of fabricating a flash memory device including the same, and more particularly to methods of forming an impurity-containing insulating film and methods of fabricating flash memory devices including such an insulating film.
One important characteristic of a flash memory device is the quality of its tunnel insulating film. This is because a flash memory generally stores or erases information by tunneling charges into a tunnel insulating film. This trait applies to both floating gate and charge trap type flash memory. A flash memory device generally uses an FN tunneling scheme when storing information. This FN tunneling scheme is a scheme for tunneling charges at a relatively high voltage, thereby typically ensuring stable tunneling. Indeed, it is important to ensure stable tunneling, but the FN tunneling scheme also may have a problem in that it is generally desired to lower a tunneling voltage as much as possible. In order to solve this problem, a study has been conducted to provide a method of lowering the energy band gap of a tunnel insulating film. Such a method is based on the fact that a voltage at which charges tunnel can be maintained lower when the energy band gap of a tunnel insulating film is lowered, and thus the stress burden of the tunnel insulating film is mitigated.
As a method of lowering the energy band gap of a tunnel insulating film, there has been proposed a method in which a tunnel insulating film is formed of a silicon oxy-nitride film. In particular, when a silicon oxy-nitride film containing high-concentration nitrogen is formed in upper and lower portions of the tunnel insulating film, there is a chance of lowering the energy band gap without significantly degrading the inherent properties of the tunnel insulating film.
In an effort to produce such results, methods have been proposed in which a tunnel insulating film is formed of a silicon oxy-nitride film by forming a silicon oxide film into which nitrogen is injected. Such technology corresponds to a method in which a tunnel insulating film is formed by a silicon oxide film on a semiconductor substrate, and then a silicon oxy-nitride film is partially formed using a plasma technique and/or a thermal reaction technique. A tunnel insulating film formed in this way has the possibility of improving FN tunneling characteristics as compared to a conventional tunnel insulating film made of only a silicon oxide film.
However, such silicon oxy-nitride film methods generally require different semiconductor fabrication equipment, which may result in a lack of continuity between the processes. Also, a problem of high process costs generally arises because the individual processes must typically be combined into one complete method. Moreover, this technology may be accompanied by surface damage due to a plasma process, and generally fails to closely control the concentration of nitrogen injected into a tunnel insulating film, so attainable characteristics may not be as good as expected.